The State Department acknowledged Tuesday, however, that world powers have yet to even meet to negotiate details.
"I can't say categorically that a week from last Thursday there must be a cessation of hostilities, but certainly we're going to expect that there is progress," State Department Deputy Spokesman Mark Toner said.
Russian and regime airstrikes against hospitals and schools in northern Syria drew sharp criticism by Washington and cast further doubt on the viability of the "cessation of hostilities" announced last week in Munich, scheduled to start Friday.
The Russian Ministry of Defense denied any responsibility for the attacks in a series of tweets on Tuesday, but Toner blamed the Russians for continuing to supply the Syrian regime and condemned the attacks as "unacceptable."
"They were Russian aircraft that carried it out, these strikes," Toner said. "We're pretty confident in our assessment that this was Russia that carried out these strikes."
He said of Moscow's conflicting messages on the fighting in Syria that,"It's put up or shut up."
He continued, "Russia has pledged ... that there should be a cessation of hostilities. They need to exert influence on the Syrian regime to make sure that happens."
The ceasefire agreement, announced Friday by Secretary of State John Kerry in Munich, calls for humanitarian aid to start flowing on Saturday to besieged Syrian areas.
But on Tuesday, Toner could not confirm that any of it was delivered. The U.N. special envoy on Syria, Staffan de Mistura, said during a visit to Damascus that he had been assured by regime officials that government forces would begin allowing humanitarian access to seven besieged cities on Wednesday.
A task force of world powers was charged with negotiating the terms of the cessation of hostilities, which Kerry said would take place on Friday, but Toner acknowledged had yet to meet.
"Obviously, there's an urgency here and we need to get this group together, but they're still working through the various issues and challenges," he said.
The Pentagon expressed a new willingness Tuesday to monitor, and potentially even enforce, the fulfillment of the agreement.
In response to a question from CNN's Barbara Starr, spokesman Peter Cook said the U.S. would take action if a violation were observed.
"We will be keeping a close eye on who abides by it and who does not," he said. "And we will be in a position to say, clearly -- and to respond, if necessary, if there are violations of that cessation of hostilities."